Molybdenum metabolism in Klebsiella pneumoniae will be further pursued with mutants unable to synthesize active FeMo-co, the active site of nitrogenase. Production of FeMo-co in vitro will be tested with the addition of molybdate and ATP to the extracts. Regulation of nitrogenase synthesis by molybdenum will take advantage of mutants unable to accumulate molybdate. The nature of the regulatory mechanism will be analyzed with nif-lac fusions as well as with mutants having defective regulatory proteins. Work will be continued in our attempts to produce crystals of FeMo-co and the Mo-Fe cluster that are satisfactory for X-ray crystallographic studies. Proteins involved with electron transport to FeMo-co also will be purified and studied.